/*
 * Copyright 2002-2007 Network Engine for Objects in Lund AB [neotechnology.com]
 * 
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as
 * published by the Free Software Foundation, either version 3 of the
 * License, or (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU Affero General Public License for more details.
 * 
 * You should have received a copy of the GNU Affero General Public License
 * along with this program.  If not, see <http://www.gnu.org/licenses/>.
 */
package org.neo4j.impl.transaction.xaframework;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;


/**
 * During recovery (after system crash) the {@link XaLogicalLog} will examine 
 * the the logical log and re-create any transaction that didn't complete. 
 * <CODE>XaCommandFactory</CODE> job is to read in commands from the logical 
 * log and re-create them.
 */
public abstract class XaCommandFactory
{
	/**
	 * Reads the data from a commad previosly written to the logical log and 
	 * creates the command again.
	 * <p>
	 * The implementation has to guard against the command beeing corrupt and 
	 * not completly written to the logical log. When unable to read the 
	 * command <CODE>null</CODE> should be returned. <CODE>IOException</CODE>
	 * should only be thrown in case of real IO failure. The <CODE>null</CODE>
	 * return is a signal to the recovery mechanism to stop scaning for more 
	 * entries.
	 *
	 * @param fileChannel The channel to read the command data from
	 * @param buffer A small buffer that can be used to read data into
	 * @return The command or null if unable to read command
	 * @throws IOException In case of real IO failure
	 */
	public abstract XaCommand readCommand( FileChannel fileChannel, 
		ByteBuffer buffer ) throws IOException;
}
